White teeth have long been considered cosmetically desirable. Unfortunately, teeth become almost invariably discolored in the absence of external intervention. The tooth materials which are generally responsible for presenting a stained appearance are enamel, dentin, and the acquired pellicle. In particular, tooth enamel is formed predominantly from inorganic material, mostly in the form of hydroxyapatite crystals, and further contains approximately 5% organic material, primarily in the form of collagen. Conversely, dentin is composed of about 20% protein, including collagen, with the balance comprising of inorganic material, predominantly hydroxyapatite crystals, similar to that found in enamel. The acquired pellicle, on the other hand, is a proteinaceous layer on the surface of tooth enamel which reforms rapidly even following an intensive tooth cleaning with highly abrasive prophylaxis pastes.
Tooth discoloration results from both extrinsic and intrinsic staining. Extrinsic staining of the tooth surface arises as a result of the accumulation of various chromogenic substances (in addition to chromogen precursors, which are initially colorless, but later chemically convert to chromogens) within the acquired pellicle. This type of staining can usually be removed by mechanical methods, which remove the acquired pellicle or portions thereof, along with the adherent chromogens. Aging of extrinsic stains, however, has been known to make the extrinsic stains less susceptible to removal by mechanical means, perhaps due to increased depth of extrinsic stain penetration into enamel over time. Such stains, therefore, require the use of chemicals, such as oxygenating agents, which can penetrate the tooth enamel to oxidize or solubilize the deep-seated chromogens. In contrast, intrinsic staining occurs as a result of chromogenic substances derived from sources within the tooth. This type of staining is not amenable to mechanical methods of tooth cleaning, and the aforementioned chemical methods are usually required.
Tooth-whitening compositions generally fall into two categories: (1) liquids, gels, or pastes, including toothpastes, that may be mechanically agitated at the stained tooth surface in order to effect tooth stain removal through abrasive erosion of stained acquired pellicle; and (2) liquids, gels, or pastes that accomplish the tooth-whitening effect by a chemical process while being in contact with the stained tooth surface for a specific period, after which the formulation is removed. In some cases the mechanical process is supplemented by an auxiliary chemical process, which may be oxidative or enzymatic.
The majority of professionally-monitored at-home tooth-whitening compositions act by oxidation. These compositions are dispensed into a custom-made tooth-whitening tray for use directly by a patient. Typically, these trays must be held in the mouth of the patient for a period of time often greater than about 60 minutes, and sometimes as long as 8 to 12 hours in order to produce any results. The slow rate of whitening is in large part the consequence of formulations that are developed to maintain stability of the oxidizing composition prior to use. These oxidizing compositions may contain a hydrogen peroxide precursor, for instance, carbamide peroxide, which is mixed with an anhydrous or low-water content, hygroscopic viscous carrier containing glycerin and/or propylene glycol and/or polyethylene glycol. When contacted by the moisture in saliva, carbamide peroxide dissociates into urea and hydrogen peroxide. Moreover, because of the slow rate of whitening with the hygroscopic carrier, the currently available tooth-whitening compositions containing carbamide peroxide can cause tooth sensitization in many patients. Tooth sensitivity is believed to result from the movement of fluid through the dentinal tubes toward nerve endings in the tooth. This fluid movement is enhanced by the carriers for the carbamide peroxide. In fact, it has been determined that glycerine, propylene glycol and polyethylene glycol can each give rise to varying amounts of tooth sensitivity following exposure of the teeth to heat, cold, overly sweet substances, and other causative agents.
Prolonged exposure of teeth to whitening compositions, as presently practiced in the industry, has a number of other adverse effects in addition to tooth sensitivity. These include: (i) solubilization of calcium from the enamel layer at pH less than 5.5 with associated demineralization, (ii) penetration of the intact enamel and dentin by the whitening agents, so as to reach the pulp chamber of a vital tooth thereby risking damage to pulpal tissue, and (iii) dilution of the whitening compositions with saliva with resulting leaching from the dental tray and subsequent digestion.
Furthermore, in situ decomposition of hydrogen peroxide in peroxide-based tooth whitening compositions can lead to the formation of free radicals such as hydroxyl and per hydroxyl radical species, which are highly reactive molecules that have been implicated in the formation of cancerous cells. Although the use of hydrogen peroxide at concentrations of up to 3% by weight in oral care products has recently been deemed safe for everyday use in the United States, many regulatory agencies throughout the world have placed an upper limit on the amount of hydrogen peroxide permitted for such applications. For instance, the European Community Cosmetic Directive, which regulates the permissible level of additives in various products, has established the upper limit for hydrogen peroxide at 0.1% by weight of an oral care composition. This level is insufficient for effecting a tooth whitening effect in a reasonable period of time.
As there is a cosmetic need for whitening teeth while avoiding the adverse effects usually associated with prolonged exposure to tooth whitener having peroxide, it is desirable to provide a non-peroxide tooth whitening composition capable of whitening the teeth in a reasonably short period of time.